1. Field of the Invention
The present invention relates to a switching power supply apparatus and a multi-output type DC/DC converter outputting a plurality of voltages from one input voltage, and more particularly to a DC/DC converter provided with a selection circuit for switching output currents according to the level of each voltage output.
2. Description of Related Art
There is a DC/DC converter as a power supply apparatus converting a direct-current input voltage into a direct-current voltage of an electric potential different from that of the input voltage to output the converted direct-current voltage. Moreover, there is a circuit shown in FIG. 9 as a multi-output type DC/DC converter outputting a plurality of direct-current voltages having different electric potentials from one direct-current input. Such a multi-output type DC/DC converter is disclosed in, for example, Japanese Patent Application Publication Laid-open No. 2005-117886.
The DC/DC converter shown in FIG. 9 turns on and off a main switch SW0 based on a pulse P1 or a pulse P2, which are output from comparators CMP1 and CMP2, respectively, and the DC/DC converter selectively turns on any one of change-over switches SW1 and SW2 at a predetermined period based on the output of a frequency divider to apply the output current of an inductor (reactor) L to a rectification & smoothing circuit 16a or 16b in a time division method. Thereby, the DC/DC converter outputs direct-current voltages Vout1 and Vout2, which have been converted to voltages having desired levels severally, from a first output terminal OUT1 and a second output terminal OUT2.
Because the multi-output type DC/DC converter as shown in FIG. 9 is configured to switch the output current of the inductor L by the time division method, the multi-output type DC/DC converter has the advantages of being capable of decreasing the number of inductors, and of being capable of attaining the miniaturization of the direct-current power supply apparatus. However, because the multi-output type DC/DC converter of FIG. 9 distributes the output current of the inductor L to the first output terminal OUT1 and the second output terminal OUT2 at a predetermined period (fixed), the multi-output type DC/DC converter has a problem of the difficulty of keeping the desired electric potentials if the current ratio of the loads connected to the respective two output terminals greatly changes.
To put it concretely, for example, there is examined the case where, in a system using a multi-output type DC/DC converter the period of the time division method of which has been designed on the supposition that the ratio of the consumption current of the load connected to the first output terminal OUT1 to the consumption current of the load connected to the second output terminal OUT2 is 2 to 1, the ratio of the consumption current of the load of the first output terminal OUT1 to the consumption current of the load of the second output terminal OUT2 is suddenly reversed to be 1 to 2. In this case, if the period of the time division method is fixed, there is the possibility of producing the situation in which the output voltage Vout2 of the second output terminal OUT2, the load of which has increased, does not reach a desired electric potential, and on the other hand in which the output voltage Vout1 of the first output terminal OUT1, the load of which has decreased, becomes higher than the desired electric potential.